Capacitive sensors have a wide range of applications, and are among others used for the detection of the presence and/or the position of conductive body in the vicinity of an antenna electrode. As used herein, the term “capacitive sensor” designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which may be identical with or different from transmitting antenna electrodes—at which the influence of an object or living being on the electric field is detected.
The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmitting electrode, building up an electric field to a receiving electrode, and the displacement current induced at the receiving electrode is measured. The measured displacement current depends on the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
The capacitive coupling strength may e.g. be determined by applying an alternating voltage signal to an antenna electrode and by measuring the current flowing from that antenna electrode either towards ground (in the loading mode) or into a second antenna electrode (in coupling mode). This current may be measured by a transimpedance amplifier, which is connected to the sensing electrode and which converts the current flowing into the sensing electrode into a voltage proportional to the current.
Capacitive sensors, which use a heating element as antenna electrode are known in the patent literature. US 2011/0148648 A1 discloses a capacitive occupant sensing system for a vehicle seat, using a seat heating element 12 as antenna electrode. FIG. 1 schematically shows an illustration of this prior art. Voltage source 2 represents the power supply for the heater, for example a seat heater control unit. Electronic control module (ECM) 1 is configured as a capacitive measurement circuit. It comprises a common mode choke 5, an AC voltage source 9 and capacitors 6, 7 and 8. Capacitor 8 couples the AC voltage generated by AC voltage source 9 into the node 11. The heating element 12 has complex impedance 13 towards ground. The complex impedance 13 includes a capacitive component as well as a resistive component, which depend on the occupancy state of the vehicle seat. Complex impedance 13 is thus hereinafter also referred to as “unknown impedance” or “impedance to be determined”. The capacitor 8 forms together with the unknown impedance 13 a voltage divider. The complex voltage Umeas between node 11 and circuit ground 10 can be used to calculate the unknown complex impedance 13. The common mode choke 5 decouples the AC voltage on node 11 from AC ground due to its large impedance. The heating element 12 may at the same time be traversed by the DC current supplied by voltage source 2 and driven with the AC voltage by the capacitive measurement circuit. Capacitors 6 and 7 ensure that a defined AC ground is present on the side of the common mode choke 5 that is connected to the DC power supply of the seat heater. Ground 3 is the reference ground. The connections of the common mode coke 5 are numbered 5.1 through 5.4: connection 5.1 connects the first winding to the high potential side of the voltage source 2; connection 5.2 connects the first winding to the high potential side of the heating element 12; connection 5.3 connects the second winding to the low potential side of the heating element 12 and connection 5.4 connects the second winding to the low potential side of the voltage source 2.
Resistor 4 represents the wiring resistance of the wiring between the low potential side of voltage source 2 and the fourth connection 5.4 of common mode choke 5. There is a similar wiring resistance for the upper wiring between the high potential side of voltage source 2 and common mode choke 5, but this can be neglected for the explanation that follows. Typically, the voltage source 2, which represents the seat heater control unit, is switched on and off periodically to control the heating power of seat heater 12 according to a pulse-width-modulation scheme. A typical switching frequency is 25 Hz, for example. Each time voltage source 2 is switched on, the current through wiring resistance 4 rises from substantially 0 A to the operating current of the seat heater, which is, for example, for a voltage source 2 voltage of 12 V, a seat heater resistance of 1Ω and a wiring resistance of 0.1 Ω equal to about 10.9 A. This current of 10.9 A generates a voltage drop of 1.09 V across wiring resistance 4 each time the voltage source 2 is switched on. This implies that the voltage on the fourth connection 5.4 of the common mode choke 5 will rise to 1.09 V, and consequently also the voltage on node 11 will rise to 1.09 V. The resistance of the second winding of common mode choke 5 is neglected here, but it will also contribute to an additional voltage drop due to its finite conductance. The voltage step of 1.09 V on the sense node may disturb the measurement of the signal voltage on sense node 11, since the step function has a wide frequency bandwidth. The situation is even worse if the seat heater control unit connected to the electronic control module 1 does not interrupt the heating circuit on the high potential side but on the low potential side. This means indeed that the node 11 experiences a voltage drop of about 12 V−1.09 V=10.91 V, which is worse than the 1.09 V step mentioned above. This situation may arise if one type of electronic control module 1 for the capacitive sensing must be usable for different types of seat heater control units e.g. for cost reasons.
U.S. Pat. No. 6,703,845 B2 discloses an occupant sensor for a vehicle seat, wherein the heating element is used either as the sensing electrode or as a driven shield electrode. In some of the described embodiments, the heating element AC-decoupled from the heating current source by inductors. FIG. 2 is a schematic illustration of the occupant sensor of U.S. Pat. No. 6,703,845 B2. The most substantial difference is that the system of U.S. Pat. No. 6,703,845 B2 uses separate inductors 14 and 15 instead of a common mode choke 5. The system of U.S. Pat. No. 6,703,845 B2 suffers from the same disadvantage discussed with respect to FIG. 1. Furthermore, experiments and simulations show that inductors exhibiting the necessary impedance to alternating current below 1 MHz will be so expensive that the solution of U.S. Pat. No. 6,703,845 B2 is unrealistic in an automotive vehicle.
WO2011/117237 discloses an occupancy sensor for a vehicle seat, which measures the complex current flowing into the heating element in response to an AC voltage applied thereto. The circuit configuration is schematically illustrated in FIG. 3. The transimpedance amplifier 17 keeps node 11 at the same AC voltage as the output of AC signal source 9. The reference input 17.1 of the transimpedance amplifier 17 is connected to the AC signal source 9. Transimpedance amplifier 17 converts the current flowing into its signal input 17.2 into a voltage on its output 18, which is indicative of the input current. Since the voltage on node 11 is known, the complex current flowing into the node 11 and, hence, the complex voltage at the output 18 of the transimpedance amplifier is indicative of the complex impedance 13. The capacitive sensing system of FIG. 3 suffers from the same problems as those of FIGS. 1 and 2, when the voltage source 2 of the heating circuit is switched on and off.